The present invention relates to valve metal powders and electrolytic capacitors using the valve metal powders as well as methods of making the powders and the capacitors. More particularly, the present invention relates to high surface area valve metal powders and capacitors having high capacitance.
Tantalum capacitors, made from tantalum powder, have been a major contributor to the miniaturization of electronic circuits and have made possible the application of such circuits in extreme environments. Tantalum capacitors typically are manufactured by compressing tantalum powder to form a pellet, sintering the pellet in a furnace to form a porous tantalum body (electrode), and then subjecting the porous body to anodization in a suitable electrolyte to form a continuous dielectric oxide film on the sintered body.
Development of powders suitable for making tantalum capacitors has resulted from efforts by both capacitor producers and tantalum processors to delineate the characteristics required for tantalum powder for it to best serve in the production of quality capacitors. Such characteristics include specific surface area, purity, shrinkage, pressability, and the like.
First, the powder should provide an adequate electrode surface area when formed into a porous body and sintered. The μFV/g of tantalum capacitors can be related to the specific surface area of the sintered porous body produced by sintering a tantalum powder pellet. The specific surface area of tantalum powder can be related to the maximum μFV/g attainable in the sintered porous body.
Purity of the powder can also be an important consideration. Metallic and non-metallic contamination tends to degrade the dielectric oxide film in tantalum capacitors. While high sintering temperatures serve to remove some volatile contaminants, high temperatures also tend to shrink the porous body reducing its net specific surface area and thus the capacitance of the resulting capacitor. Minimizing the loss of specific surface area under sintering conditions, i.e., shrinkage, is necessary in order to produce high μFV/g tantalum capacitors.
As discussed above, the μFV/g of a tantalum pellet can be a function of the specific surface area of the sintered powder. Greater net surface area can be achieved, of course, by increasing the quantity (grams) of powder per pellet; but, cost and size considerations have dictated that development be focused on means to increase the specific surface area of tantalum powder.
One proposed method for increasing the specific surface area of tantalum powder is flattening the powder particles into a flake shape. However, efforts to increase specific surface area by making thinner tantalum flakes have been hindered by concomitant loss of processing characteristics, for example, very thin tantalum flake would be expected to have poor pressability and low forming voltages, for example. Also, in processes to making high surface area powders, the milling can take many hours which can be time consuming, expensive, and the long milling times typically result in reaching a point where the powder fractures. Thus, there has been somewhat of a threshold which has prevented high capacitance powders until the present invention.